Sonar circumferential flow conditioner

ABSTRACT

Methods and apparatus enable measuring flow of a fluid within a conduit. For example, flowmeters may measure the velocity of production fluid flowing through production pipe of an oil/gas well. The flowmeters rely on detection of pressure variations generated as a result of a backward-facing step as a basis for flow measurement calculations. Pressure sensing occurs away from the step in a direction of the flow of the fluid in an enhanced turbulence region of the flowmeter where the inner diameter remains enlarged as a result of the step.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 12/035,578filed Feb. 22, 2008, now U.S. Pat. No. 7,607,361, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to flow measurementapparatus and methods.

2. Description of the Related Art

In the petroleum industry, as in many other industries, ability tomonitor flow of fluids in process pipes offers considerable value. Someapproaches to determining flow rate of a fluid utilize a meter, referredto as a Sonar flowmeter, to detect and analyze pressure variationstraveling with the fluid. However, these pressure variations due tosensitivity of the meter and signal-to-noise ratio limitations may notbe detectable under some operating conditions.

For example, dynamic pressures of the flow not being above a certainminimum threshold can limit ability to detect the pressure variations.Since the dynamic pressure is based on velocity of the flow and densityof the fluid, the meter may not enable measuring flow rates as slow asdesired, especially for gas-rich fluids that have relatively lowerdensities. Further, changes in flow regimes such as occur through anozzle can cause reductions of turbulence within the flow as the flowaccelerates through the nozzle and tends to laminarize. Due toturbulence being what provides the pressure variations detected by themeter, structures that change the flow regime and may be beneficial ornecessary for other reasons can adversely impact ability to detect thepressure variations, which may already be weak depending on the dynamicpressure of the flow. Redeveloping of a turbulent boundary layer occursaway from the structure that changed the flow regime. However, locationof the meter where turbulent structures redevelop increases a lengthrequirement for the meter, thereby necessitating greater installationspace that may not be available in some situations, such as on offshorerigs. This excess length also adds to weight making transport moredifficult and causes the meter to be more expensive and time consumingto manufacture.

Therefore, there exists a need for improved apparatus and methods forsensing flow velocity of a fluid based on detecting pressure variationstraveling with the fluid.

SUMMARY OF THE INVENTION

An apparatus in one embodiment detects pressure variations within afluid that is flowing. A conduit of the apparatus contains the fluid andhas a first section with a first inner diameter and a second sectionwith a second inner diameter larger than the first inner diameter todefine between the first and second sections a backward-facing step thatproduces the pressure variations. An array of pressure sensors sense thepressure variations and are spaced along a length of the second sectionwith the second inner diameter.

For one embodiment, a method of detecting pressure variations within afluid that is flowing in a conduit permits measurement of a velocity ofthe fluid. The method includes introducing the pressure variations intothe fluid with a backward-facing step formed by an enlargement in aninner diameter of the conduit in a direction of fluid flow. Further,sensing the pressure variations with an array of pressure sensors atspaced locations along a length of the conduit where the inner diameterremains enlarged past the step occurs prior to calculating the velocityof the fluid based on the pressure variations that are sensed.

A system detects pressure variations within a fluid for measuring flowof the fluid, in one embodiment. The system includes a conduit thatcontains the fluid and is configured such that a first section of theconduit is located between a second section of the conduit and aconverging inner diameter portion of the conduit that gradually reducesa primary inner diameter of the conduit to a first inner diameter at thefirst section, which first section transitions to the second sectionthat has a second inner diameter larger than the first inner diameter.The system further includes an array of pressure sensors to sense thepressure variations and signal interface circuitry configured to measurea velocity of the fluid based on signals from the array of pressuresensors spaced along a length of the second section with the secondinner diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a cross-section view of a conduit with a backward-facing stepdisposed ahead of sensors of a flowmeter coupled to the conduit todetect pressure variations of fluid flowing through the conduit, inaccordance with embodiments of the invention.

FIG. 2 is a cut-away view of a conduit with a circumferential flowconditioner disposed between sensors of a flowmeter and a flow nozzle,according to one embodiment of the invention.

FIG. 3 is a partial cross-section view of a flow rate measuring systemutilizing the circumferential flow conditioner shown in FIG. 2.

FIG. 4 is a graph of sonar velocity for flowmeters with and without abackward-facing step versus a reference velocity.

FIG. 5 is a graph for flowmeters with and without a backward-facing stepof a quality metric versus sonar dynamic pressure.

FIG. 6 is a graph of sonar over-reading for flowmeters with and withouta backward-facing step versus liquid mass quality.

DETAILED DESCRIPTION

Embodiments of the invention generally relate to flowmeters, such asused to measure the velocity of production fluid flowing throughproduction pipe of an oil/gas well. The production fluid may containliquid and/or gaseous phases of hydrocarbons and/or water. In addition,the flowmeters may couple to conduits conveying fluids associated withother industries. The flowmeters rely on detection of pressurevariations generated as a result of a backward-facing step as a basisfor flow measurement calculations. Pressure sensing occurs away from thestep in a direction of flow of the fluid in an enhanced turbulenceregion of the flowmeter where the inner diameter remains enlarged as aresult of the step.

FIG. 1 illustrates a flowmeter 200 with sensors 202 distributed along alength of a conduit 204 and coupled to the conduit 204 to detectpressure variations of fluid 206 (depicted by an arrow showing flowdirection) flowing through the conduit 204. Spacing between the sensors202 enables sensing the pressure variations that travel with flow of thefluid 206 and that are induced into the flow as described furtherherein. For some embodiments, the sensors 202 non-intrusively measurethe pressure variations with respective coils of optical fiber wrappedaround the conduit 204 to define each of the sensors 202. Other pressuremeasuring devices such as piezoelectric or Polyvinylidene Fluoride(PVDF) based detectors can form the sensors 202 and provide pressuretime-varying signals for the flowmeter 200. Interpretation of thepressure signals from the sensors 202 enables determination of at leastthe mixture flow velocity of the fluid 206. U.S. Pat. No. 6,782,150,which is herein incorporated by reference, describes examples ofappropriate calculations for determining the velocity with similarapparatus.

The conduit 204 includes a backward-facing step 208 disposed ahead ofthe sensors 202. As shown, the backward-facing step 208 occurs between afirst section 210 of the conduit 204 and a relatively larger innerdiameter second section 212 of the conduit 204 with direction of theflow of the fluid 206 progressing from the first section 210 to thesecond section 212. For some embodiments, the step 208 defines, relativeto the flow direction, a ninety degree transition between innerdiameters of the first and second sections 210, 212 around an entirecircumference of the conduit 204. Angle of transition may vary so longas separation disturbances 214 are generated for detection by thesensors 202. The separation disturbances 214 occur as a result of thefluid 206 flowing across the step 208 at which point turbulent eddiesdevelop to fill in behind the step 208. The sensors 202 detect theseparation disturbances 214 in the second section 212 of the conduit 204that has the larger inner diameter than the first section 210. In someembodiments, the inner diameter of the second section 212 remainsconstant from the step 208 to across where the sensors 202 are disposed.

No requirement exists for the separation disturbances 214 to have afixed frequency for any particular flow rate. In contrast, a vortexshedding meter for example utilizes shedding vortices in whichperiodicity is important. The shedding vortices require particularlyshaped bluff bodies within the flow path of the meter to produceconvection of the shedding vortices at a predictable certain rate. Thebluff bodies are purposefully shaped to produce pressure balancedvortices symmetrical about the bluff bodies to ensure a singlepredictable frequency of vortices are shed for a given rate of flow offluid within the meter. The backward-facing step 208 may provide at anysingle given flow rate a range of frequencies for the separationdisturbances 214 in some embodiments. Analysis of the signals from thesensors 202 may correspond to multiple frequencies associated with theseparation disturbances 214 to thereby provide data that may be desiredfor use in the aforementioned interpretation of the signals. By way ofexample, the step 208 being continuous and uniform around the entirecircumference of the conduit 204 lacks any counterbalancing feature suchthat no particular periodicity is generated.

Upon being introduced into the fluid 206 as a result of a separationeffect at the step 208, the separation disturbances 214 travel with theflow. The step 208 protects the separation disturbances 214 convectingpast the sensors 202 for an enhanced turbulence region 216 past the step208 where coherence endures prior to downstream re-development of flowcharacteristics without the separation disturbances 214. In particular,fluid within areas in the second section 212 outside of a central regioncorresponding to the smaller diameter of the first section 210 remainout of alignment with the flow coming from the first section 210 suchthat outside the central region tends to be protected at least for theenhanced turbulence region 216. This phenomenon associated with the step208 is in contrast to a groove that lacks any such protection forpotential unsteady flow fields of certain frequencies (e.g., Rossiterfrequencies) generated as the flow travels across front and back wallsof a cavity provided by the groove within an otherwise uniform path. Inaddition to the mechanism and way the separation disturbances 214 areintroduced with the step 208 being different from the groove approach,the step 208 can, if desired, provide the separation disturbances 214also without introducing restrictions in the conduit 204 since anyprotrusions can limit flow, inhibit passage through the conduit 204 andpotentially be eroded or worn away.

For some embodiments, the enhanced turbulence region 216 includes thesensors 202 and extends axially up to about 10 or 20 diameter lengthsdownstream from the step 208 (based on the inner diameter of the secondsection 212). Height of the step 208 may vary based on application anddiameter of the conduit 204. For example, the height of the step 208compared to the diameter of the conduit 204 at the first section 210 mayprovide a ratio of about 0.015, as the height of the step 208 may bebetween 1.0 millimeter (mm) and 10.0 mm or about 3.0 mm while thediameter of the conduit 204 at the first section 210 may be between 25.0mm and 500.0 mm or about 50.0 mm.

FIG. 2 shows a cut-away view of a conduit 108 with a circumferentialflow conditioner 128 (e.g., a backward-facing step) disposed betweensensing elements 122-124 of a pressure sensor array (Sonar) based meter104 (shown in FIG. 3 by example within a flow rate measuring system 100)and a flow nozzle or converging inner diameter portion 120 of theconduit 108. A fluid flow 110 ahead of the converging inner diameterportion 120 includes turbulence 135 that at least diminishes upon thefluid flow 110 passing through the converging inner diameter portion 120of the conduit 108. The converging inner diameter portion 120 graduallyreduces the inner diameter of the conduit 108 from a primary diameter D,thereby resulting in acceleration and laminarization of the fluid flow110. Thereafter, the flow conditioner 128 defines a transition in thedirection of the fluid flow from a first inner diameter d₁ to a secondinner diameter d₂ that is larger. The primary diameter D may be largerthan the second inner diameter d₂ and may be about twice the first innerdiameter d₁. The difference between d1 and d2 determines the stepheight. Variation in the step height can accommodate moderate changes inthe nozzle geometry (i.e., the ratio d1/D, also known as the nozzle Betaratio) to better match the ranges of existing differential pressuremeasurement transducers to the flow rates required by the application.The sensing elements 122-124 can detect, based on strain on the conduit108 at locations where the conduit 108 retains the second inner diameterd₂, pressure variations caused by separation disturbances 134 producedas a result of the flow conditioner 128.

FIG. 3 illustrates an exemplary flow rate measuring system 100 utilizingthe circumferential flow conditioner 128. The flow rate measuring system100 includes a Venturi-based meter 102, the pressure sensor array(Sonar) based meter 104, and an optional water-in-liquid ratio (WLR)meter 106, all disposed along the conduit 108 containing the fluid flow110. The meters 102, 104, 106 couple to signal interface circuitry 112through a transmission line 114. The signal interface circuitry 112receives and processes signals from the meters 102, 104, 106 tocalculate velocity and/or phase fraction flow rates of the fluid flow110 using logic based on principles described for example in U.S. patentapplication Ser. No. 11/625,460 entitled “Wet-gas Flowmeter,” which isherein incorporated by reference in its entirety.

The Venturi-based meter 102 includes first and second ports 116, 118exposed to pressures of the fluid flow 110 that traverses a constrictionformed by the converging inner diameter portion 120 of the conduit 108.For some embodiments, the first and second ports 116, 118 tap into theconduit 108 respectively at where there is the primary diameter D andthe first inner diameter d₁. The Venturi-based meter 102 defines adifferential pressure sensing meter between the first port 116 disposedupstream of the converging inner diameter portion 120 and the secondport 118 located in a throat section downstream of the converging innerdiameter portion 120. The fluid flow 110 tends to laminarize uponpassing through the converging inner diameter portion 120, therebyreducing turbulence that may be needed to achieve suitable results withthe Sonar-based meter 104.

The Sonar-based meter 104 can include two or more pressure sensingelements along the enhanced turbulence region. The maximum number ofsensors is limited by the physical width of each sensing element and thelength of the enhanced turbulence region. Three of the pressure sensingelements 122, 123, and 124 are axially distributed along a length of theconduit 108. Proper spacing between the sensing elements 122-124 enablessensing short-duration local pressure variations traveling with thefluid flow (referred to as “flow velocity sensing”) and can also enablesensing acoustic signals traveling at the speed of sound through thefluid flow 110 within the conduit 108 (referred to as “acousticsensing”). Interpretation of these signals from the Sonar-based meter104 enables determination of at least the mixture flow velocity (v_(m))of the fluid flow 110 and may also enable determination of the speed ofsound (SOS, a_(mix)) of the fluid flow 110.

The WLR meter 106 can operate based on principles of spectroscopy byrelying on differences in absorption between oil and water ofnear-infrared light. In some embodiments, an intrusive probe of the WLRmeter 106 within the fluid flow 110 provides a sample region 126 inwhich input light passes through a portion of the fluid flow 110 and isdetected thereafter. Absorption of the input light by the fluid flow 110attenuates the input light and depends in a wavelength conditionedmanner on the contents of the fluid flow 110 to enable determining, forexample, the percentage of water and the percentage of oil.

FIGS. 4-6 show results obtained from experiments conducted utilizing twosimilar flowmeters, such as the flow rate measuring system 100 shown inFIG. 3, but with only one of the two having a backward-facing step. Agraph provided in FIG. 4 plots Sonar velocity determined with theflowmeters with and without the backward-facing step versus a referencevelocity. A reference line 400 identifies the reference velocity. Firstand second lines 401, 402 correspond to data obtained using respectivelythe flowmeter with the backward-facing step and the flowmeter withoutthe backward-facing step. Presence of the step only minimally affects(by about 5%) the assessed velocity based on the difference between thefirst and second lines 401, 402. Moreover, the first line 401 retainslinearity such that the flowmeter with the backward-facing step may becalibrated similar to the flowmeter without the backward-facing step.

FIG. 5 depicts a graph of a quality metric versus sonar dynamic pressurefor the flowmeter with the backward-facing step (a first curve 501) andthe flowmeter without the backward-facing step (a second curve 502).Dynamic pressure is calculated based on the bulk velocity that prevailsalong the Sonar sensor array. The quality metric is a quantification ofperformance for a result of a flow array processing algorithm. Thealgorithm result V is the maximum of a power correlation function P(v).The flow quality metric Q is calculated by evaluating the powercorrelation function for a given velocity V and its negative, thenforming a ratio of their difference and their sum:

$Q = {\frac{{P(V)} - {P\left( {- V} \right)}}{{P(V)} + {P\left( {- V} \right)}}.}$Random, uncorrelated noise is direction independent. However, the energyassociated with the flow vortices is correlated and moving in onedirection. The quality metric measures the asymmetry of the powercorrelation function at a given velocity. If the quality metric is nearzero, then the energy is roughly symmetrical, and there is littlecontrast between the vortical energy and the background noise. If theabsolute value of the quality metric is near one, then the energy islarger in the direction of flow. Confidence in the array processingalgorithm result may depend on the quality metric not falling below a(configurable) threshold. As it regards the backward facing step, thequality metric is a replacement for a signal to noise ratio inevaluating relative signal strength, measuring the “visibility” of thepower correlation ridge.

The quality metric for the first curve 501 increases from about 0.1 atdynamic pressures of about 100 pascals (Pa) to about 0.7 for dynamicpressures of about 3500 Pa, whereas the quality metric indicated by thesecond curve 502 is lower by a factor of about 4 to 5 and is below 0.1at 500 Pa and only increases to about 0.2 for dynamic pressures of about3500 Pa. Lower dynamic pressures can thus be measured with accuracysince any reading above a threshold for the quality metric (e.g., 0.1)may provide reliable results. As shown, the flowmeter having thebackward-facing step increases the dynamic range at minimum dynamicpressures by a factor of at least three compared to readings of similarquality made using the flowmeter without the backward-facing step.Further, the flowmeter with the backward-facing step thereby producesmore reliable readings to enable higher confidence levels of all themeasurements. For the typical material (stainless steel) and pipethickness used in commercial sonar meters, utilizing the backward-facingstep in the flowmeter may enable measurements with dynamic pressuresbelow 500 Pa and as low as 100 Pa or 50 Pa. In other words, thebackwards-facing step enhances turbulent pressure fluctuations. Saidenhanced fluctuations have an amplitude that corresponds to a higherpercentage of the dynamic pressure. For reference, the dynamic pressureof 100 Pa corresponds to water flow velocities of about 0.3 meters persecond (m/s) in comparison to only 1.0 m/s for the dynamic pressure of500 Pa. This increased detection ability with respect to minimumvelocities can open up the possibility of using the flowmeters describedherein in applications such as flare gas flow metering where the fluidflow includes flare gases.

FIG. 6 illustrates a graph of sonar over-reading for the flowmeter withthe backward-facing step (a first curve 601) and the flowmeter withoutthe backward-facing step (a second curve 602) versus a liquid massquality. The sonar over-reading is a characteristic of the flowmeters togenerate, in an increasing manner relative to higher liquid content,measured velocities higher than actual values under conditions where theamount of liquid is less than 5%. The liquid mass quality refers to theratio of the liquid to the total mass flow rates. The first and secondcurves 601, 602 (within 5% of one another) possess substantially thesame response and can thus allow compensation in a like manner.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A system for detecting pressure variations within a fluid formeasuring flow of the fluid, comprising: a conduit for containing thefluid, wherein a first section of the conduit has a first innerdiameter, a second section of the conduit has a second inner diameterlarger than the first inner diameter to define between the first andsecond sections a backward-facing step that produces the pressurevariations, and the first section of the conduit is located between thestep and a converging inner diameter portion of the conduit thatgradually reduces a primary inner diameter of the conduit to the firstinner diameter at the first section; an array of sensing elements tosense the pressure variations, wherein the sensing elements are spacedalong a length of the second section with the second inner diameter; andsignal interface circuitry configured to measure a flow velocity of thefluid based on signals from the array of sensing elements.
 2. The systemof claim 1, wherein the step defines a ninety degree angle of transitionbetween the first and second inner diameters of the first and secondsections, respectively.
 3. The system of claim 1, wherein the converginginner diameter portion is a nozzle.
 4. The system of claim 1, whereinthe array of sensing elements senses strain on the second section of theconduit as the pressure variations travel in the fluid.
 5. The system ofclaim 4, wherein the array of sensing elements comprises coils ofoptical fiber wrapped around the second section of the conduit.
 6. Thesystem of claim 4, wherein the array of sensing elements comprisespiezoelectric-based detectors.
 7. The system of claim 1, wherein thearray of pressure sensors is configured to sense acoustic signalstraveling at the speed of sound through the fluid contained within theconduit.
 8. The system of claim 1, further comprising a differentialpressure sensing meter to measure a difference in pressure between firstand second ports exposed to the fluid and traversing at least part ofthe converging inner diameter portion of the conduit, wherein the firstport is located upstream of the second port.
 9. The system of claim 8,wherein the first port is disposed upstream of the converging innerdiameter portion where the conduit has the primary inner diameter. 10.The system of claim 9, wherein the second port is disposed downstream ofthe converging inner diameter portion in the first section where theconduit has the first inner diameter.
 11. The system of claim 8, furthercomprising a water-in-liquid ratio (WLR) meter.
 12. The system of claim1, further comprising a water-in-liquid ratio (WLR) meter.
 13. Thesystem of claim 1, wherein the primary inner diameter is larger than thesecond inner diameter.
 14. The system of claim 1, wherein the shape ofthe backward-facing step is uniform and continuous around an entirecircumference of the conduit.
 15. An apparatus for detecting pressurevariations within a fluid that is flowing, comprising: a conduit forcontaining the fluid, wherein a first section of the conduit has a firstinner diameter, wherein a second section of the conduit has a secondinner diameter larger than the first inner diameter to define betweenthe first and second sections a backward-facing step that produces thepressure variations, and wherein a ratio of a height of the step and thefirst inner diameter is about 0.015; and an array of pressure sensors tosense the pressure variations, wherein the sensors are disposed along alength of the second section with the second inner diameter.
 16. Theapparatus of claim 15, wherein the height of the step is between about1.0 mm and 10.0 mm.
 17. The apparatus of claim 15, wherein the stepdefines a ninety degree angle of transition between the first and secondinner diameters of the first and second sections, respectively.
 18. Theapparatus of claim 15, wherein the shape of the backward-facing step isuniform and continuous around an entire circumference of the conduit.19. The apparatus of claim 15, wherein the first section of the conduitis located between the step and a converging inner diameter portion ofthe conduit that gradually reduces a primary inner diameter of theconduit to the first inner diameter at the first section.
 20. Theapparatus of claim 15, wherein the array of pressure sensors comprisescoils of optical fiber wrapped around the second section of the conduit.